1. Field of the Invention
The present invention relates to a gain equalizing device suitable for use in an optical transmission system for transmitting wavelength division multiplexed (WDM) signal light including a plurality of channels of optical carriers having different wavelengths.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, an optical amplifier for directly amplifying signal light has been put to practical use. An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping (exciting) the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light.
For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 .mu.m band or a 1.48 .mu.m band, a gain band including a wavelength band of 1.55 .mu.m can be obtained. Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In an optical transmission system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. At the receiving end, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by a gain characteristic (wavelength dependence of gain) which is represented by a gain deviation or gain tilt. For example, in an EDFA, a gain deviation is produced at wavelengths in the vicinity of 1.55 .mu.m. When a plurality of EDFAs are cascaded to cause accumulation of gain deviations, an optical SNR (signal-to-noise ratio) in a channel included in a band giving a small gain is degraded.
To cope with the gain deviation of an optical amplifier, an optical equalizing module incorporating an optical equalizing filter is used. Before a degradation in optical SNR in a certain channel becomes excessive due to accumulation of gain deviations, gain equalization is performed by the optical equalizing module provided at a suitable position. In a WDM long-haul optical transmission system, a gain equalizing device incorporating a plurality of such optical equalizing modules is used. A conventional gain equalizing device is configured so that a pressure-resistant housing unit for an optical submarine repeater is utilized because emphasis is placed on general versatility of a housing, and a plurality of optical equalizing modules are mounted in the pressure-resistant housing unit.
Such a pressure-resistant housing unit for an optical submarine repeater is complex in structure because it has a buffer, insulator, etc. for protecting a repeater circuit unit. Further, a gain equalizing device used in a WDM long-haul optical transmission system to be developed in the future must be adjusted so as to match the amplification characteristic of a repeater and the transmission characteristic of an optical cable.
It is accordingly necessary to first connect a plurality of optical submarine repeaters through optical cables in the field, next select an optical equalizing module having an optimum characteristic, and then connect the selected optical equalizing module to an optical fiber constituting a transmission line. In other words, it is required to develop a gain equalizing device including a plurality of optical equalizing modules having different characteristics which can be connected to one optical fiber transmission line.
In the case of an optical cable accommodating two systems, each system is required to have two optical fibers for up and down channels, so that totally four optical fibers are included in the optical cable. In the conventional gain equalizing device applied to two systems, four optical equalizing modules respectively connected to four optical fibers are mounted in the pressure-resistant housing unit. However, to provide a gain equalizing device precisely matching the amplification characteristic of an optical submarine repeater and the transmission characteristic of a cable, it is desirable to provide a plurality of optical equalizing modules having different characteristics for one optical fiber and select one optical equalizing module having an optimum characteristic from these optical equalizing modules.